Technical Abstract:
Whether a required Salmonella test series is passed or failed depends not only on the presence of the bacteria, but also on the methods for taking the samples, the methods for culturing the samples, and the statistics associated with the sampling plan. A spreadsheet program was used to perform a Monte Carlo simulation comparing the pass-fail probabilities of the two-class attribute sampling plans used for testing chilled broiler carcasses in the United States and Europe. HACCP tests in the United States use 400-ml whole carcass rinses, with 30 ml cultured for Salmonella. Twelve is the maximum number of positives out of 51 samples for passing the sample set and there are milder consequences for as few as 7 positives. The numbers (n=51 samples and c=12 positives) were chosen so that a plant operating with a Salmonella prevalence of 20%, the national baseline result for broiler carcasses sampled before the introduction of HACCP, would have an approximately 80% probability of passing the test series with 12 or fewer positive results. The European Union (EU) requires taking neck skin samples of approximately 8.3 g each from 150 carcasses, with the necks skins cultured in pools of three and with seven positives as the maximum passing score. For each sampling plan, 100,000 complete sampling sets were simulated using the random number generator in the spreadsheet. Under HACCP rules using 20% as the mean prevalence rate, 79.3% of the sample sets passed with 12 or fewer positive carcasses per set of 51, very near the expected 80% rate. A Salmonella prevalence of 9.5% on individual carcasses gave a 79.3% probability of passing with 6 or fewer positive carcasses. Under the EU test conditions, Salmonella prevalence of 3.96% in individual neck skin samples yielded a passing rate of 79.1%. Without testing of the different sampling methods and different cultural procedures using appropriately processed carcasses, it is not possible to say how a 20% Salmonella incidence in whole carcass rinses compares to a 3.96% incidence in neck skin samples.